Alternating current (AC) systems as well as many direct current (DC) power electronic systems have bidirectional current flow through a single power distribution bus. Commonly, these systems supply energy to and from electrical storage elements. Other AC and DC systems operate between active power sources and/or potential power sources. In these cases, a fault can result in an over-current condition in either direction in the system. Some examples of these systems include hybrid electric vehicle drives, grid-tie photovoltaic inverter systems, and bidirectional DC-DC converters. Such systems cover a wide range of power levels, generally require fault protection, and benefit further from bidirectional fault protection.
For distributed and OFF-grid power systems and hybrid-electric ground vehicle power systems, power electronic converters and power distribution equipment operating at up to several hundred volts and up to hundreds of kilowatts are being developed. To prevent damage to converters or other system components during fault conditions, fault current interrupt speeds in tens to hundreds of microseconds are necessary. In many of these systems, AC and DC power components operate between two voltage busses having independent sourcing capability, and require bidirectional fault isolation. Such conditions can require fault protection systems having symmetric ratings for bidirectional voltage blocking in the OFF-state and bi-directional current conduction in the ON-state.
In many applications, bidirectional fault isolation is provided by mechanical contactors or relays. However, mechanical fault protection devices often do not have adequate actuation speeds for protection of solid-state system components. Furthermore, these mechanical devices can suffer severe degradation, dramatically reducing operating life, or resulting in catastrophic system failure.
While bidirectional solid-state fault protection devices exist, they do not provide full functionality when applied to direct current and voltage, while also providing low conduction losses and self-triggering at low conduction voltage drops. For instance, commercially available bidirectional solid-state circuit breakers are designed for AC and voltage and generally use thyristors that stop current following an AC zero crossing. Other approaches using field effect semiconductor devices can have higher conduction losses resulting from more than two power transistors connected in series (such as a cascode configuration), and higher turn-OFF thresholds at high currents.